The research program is directed towards appraising the role of heritability in mental development from birth to 8 years of age. The data are drawn from 300 twin pairs (plus their siblings) who were recruited at birth and have been tested periodically throughout childhood. Heritability is expected to be a significant contributor to mental development at all ages, but its influence may be modified by prenatal and birth effects, and by the comparative advantages or disadvantages afforded by the home environment. The proposed research will deal directly with the differential influence of heritability by examining the course of mental development throughout childhood for advantaged vs. disadvantaged twins, and to identify the capabilities that are most affected. The project also focuses on the role of heritability in school-age IQ as this may be manifested in the overall IQ score, in the pattern of subtest scores, and in the consistency of the subtest pattern over ages. The results will disclose whether some components of intellectual functioning are more heavily influenced by heritability, while others are most susceptible to home- and school-environmental influences.